Here, a mobile which installs a camera functioning as an imaging device is called a mobile A, and a mobile which is a subject to be detected is called a mobile B. As a method which can detect the mobile B by analyzing an image picked up by the camera installed on the mobile A, there has been known a method based on an optical flow, disclosed in Patent Document 1. Also, the mobile B can be detected by using an image processor disclosed in Patent Document 2 with which an object at rest also can be detected. With the invention disclosed in Patent Document 2, a candidate area is set for judging whether a target object is pictured in an image picked up, a virtual plane and a motion parameter of the target object are variously assumed in a real space, an image in the candidate area at a prescribed time after is estimated and created to be compared with an image actually observed, and a presence of the target object in the candidate area is judged based on a facing direction of a virtual plane which gives an estimated image that is most similar to the image actually observed.    Patent Document 1: Japanese Unexamined Patent Publication 2004-355082 (FIG. 1)    Patent Document 2: Japanese Patent Number 3742759 (FIG. 1)